The 3rd-Generation Partnership Project (3GPP) is developing specifications for network-based management of all wireless services under the IP Multimedia Subsystem umbrella. An important goal of this project is to support the delivery of consistent services to a user regardless of whether the user is attached to a wireless network in the circuit-switched (CS) domain or in the packet-switched (PS) domain. This centralization of service management, called IMS Centralized Services, or ICS, provides a number of features for users, including support for multiple active devices associated with a single user, handling of legacy GSM devices in addition to IMS-capable devices, and standardized support for supplementary services such as call forwarding and conference calling. ICS also supports various types of media, such as video calling. With ICS, media streams can be added to or removed from ongoing sessions.
The architectural requirements for ICS are specified by 3GPP in 3GPP TS 23.292, “3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; IP Multimedia Subsystem (IMS) centralized services; Stage 2 (Release 10),” v 10.2.0, September 2010. As described in that document, an ICS-enabled mobile terminal (user equipment, or UE, in 3GPP terminology) accessing a wireless network in the CS domain (or accessing a PS access network that does not support the full-duplex speech/video component of an IMS service) establishes only a single circuit-switched call, regardless of how many ICS sessions are established for the mobile terminal (using Gm signaling). The circuit-switched call serves as the sole media bearer, and is thus shared by all the sessions. At any given time, the circuit-switched call is associated with the active ICS session.
When the ICS mobile terminal is operating in a GERAN network with dual-transfer mode (DTM) support, or if the ICS mobile terminal is operating in a UTRAN network, the mobile terminal can transmit and receive packet-switched Internet Protocol (IP) data while simultaneously maintaining a circuit-switched bearer. Thus, in these scenarios, the mobile terminal can use SIP-based signaling (at the Gm reference point) to manage the ICS sessions. However, if the network steers the ICS mobile terminal to a GERAN network without DTM support then the mobile terminal is no longer able to use Gm signaling to manage the sessions, since simultaneous transport of IP data and maintenance of a circuit-switched call becomes unsupported. In such a case, the circuit-switched call is kept, while IP transport is suspended.